Microwave electronic packages are usually produced from aluminum alloys due to low weight and good thermal dissipation. However, one of the major disadvantages of aluminum alloys is their high expansion. The electronic components mounted on a substrate, usually ceramic, are placed inside the metallic enclosure. The high expansion of aluminum alloys creates stress in the ceramic which could result in cracking of the ceramic. Typically, the industry uses epoxy to attach ceramic to a metal base. Epoxy offers sufficient compliancy but only for small packages, such as 2".times.3" type. As the package size keeps growing, the expansion mismatch becomes unacceptable.
Also, aluminum alloy packages offer significant difficulties in finally attaching a cover to the enclosures in a hermetic fashion. This is because aluminum and its alloys do not have good solderability, brazeability and weldability. Due to this same drawback, the attachment of prefabricated glass feed-throughs is also difficult which results in loss of hermeticity during manufacture or in use due to thermal fluctuations.
In the current art of building microwave packages, high strength aluminum alloys like 6061 T6 are machined to produce a package consisting of a base, side walls and holes drilled in the side walls for installing feed-throughs for wire leads. The feed-throughs consist of a Kovar lead, with glass sealed to the inner diameter of a Kovar ring. The glass sealing in the feed-throughs electrically separates the leads from the body, as well as ensuring a hermetic seal. The microwave packages are typically electroplated with silver or gold with a nickel underlayer. The electroplating serves the purpose of providing solderability/brazeability to aluminum surface and prevents the corrosion of the aluminum. These machined and electroplated packages are then installed with feed-throughs by using 80% gold-20% tin braze alloy at 325.degree. C. to braze the inside of the plated hole and outside of the Kovar ring of the feed-through. An electroplated seal ring consisting of Kovar or stainless steel is also brazed to provide seam sealing capability for the cover, at the same time the feed-throughs are brazed.
The package now is ready to accept a circuit carrying ceramic card. In most cases, the ceramic cards are soldered or epoxied onto a carrier plate prior to epoxying/soldering the carrier plate inside the package. The carrier plates minimize the effect of expansion mismatch between the package and the ceramic card and tend to prevent the warping or cracking of the ceramic card or in some cases the detachment of ceramic card from the base of the package. This is because a aluminum and its alloys have a high coefficient of thermal expansion (22 ppm/.degree.C.) compared to the ceramic card at 7 ppm/.degree.C. The carrier plates consisting of Kovar (7 ppm/.degree.C.) or stainless steel (12 ppm/.degree.C.) offer a compromise situation and hence are used as transition materials. The disadvantages, of the type of carrier plates are excess weight and a significant thermal barrier as both stainless steel and Kovar are of low thermal conductivity. Also, the brazing of Kovar for the stainless steel seam sealing ring presents a problem due to voids, leaching of electroplated material at brazing temperature etc., often resulting in leaky packages.
In a recent development, some package manufacturers have attempted to build microwave packages from silicon carbide filled aluminum metal matrix materials. These metal matrix materials offer reduced coefficient of thermal expansion (8-12 ppm/.degree.C.). However, due to carbide particle impregnation, the machining is not possible by conventional tools and techniques. Also, the question of the seam sealing ring is not resolved. In fact, use of exotic seam sealing techniques like laser and electron beam welding are inappropriate due to blow outs and uneven welding resulting from the beam striking carbide particles.
Electronic packages are usually produced from monolithic metals and alloys. The materials chosen are such that they offer reasonable compromise of thermal dissipation expansion match with ceramics, strength, reduced weight and glass sealability. In packages requiring high power dissipation or large size, these compromises are unacceptable. Such packages are then produced by choosing different metals and alloys as different components of the package and brazing them together to produce a configuration well suited to specific needs. Such packages, however suffer from poor integrity and high cost. Integrity is affected by long term unreliability of brazed joints which compromise the hermetic seal of the packages.
In order to improve package performance, the materials industry has been developing metal matrix materials. These materials are metals having non-metallic particles homogeneously dispersed through them, creating unique characteristics in the material. These particles can be graphite or ceramic, such as silicon carbide, baron, nitride, etc. which are impregnated within copper and aluminum or other alloys. The particles can be in fiber or particulate form. The principle behind the metal matrix materials is that the non-metals or ceramics, which generally have lower coefficient of thermal expansion and in some cases higher thermal conductivity, restrict the expansion of the parent metal or alloy. The amount of non-metal or ceramic loading determines the final characteristics of the product and hence can be tailored to a specific need.
The disadvantage, however, is that these materials are not solderable, brazeable or weldable. Resistance welding will not obtain a good joint due to high thermal conductivity of the composite. Laser welding causes dissociation of non-metal or ceramic particulate when the laser beam strikes it. This dissociation results in formation of gases which are entrapped within the weld resulting in loss of hermeticity and unreliable joint.
In the current art of building electronic packages, copper or aluminum alloys are machined to produce a package consisting of a base, side walls and holes drilled in the side walls for installing feed-troughs. The feed-throughs consist of a Kovar lead, with glass sealed to the inner diameter of a Kovar or nickel-iron alloy ring. The glass sealing in the feed-throughs electrically separates the leads from the body, as well as insuring a hermetic seal. The packages are typically plated with silver or gold with a nickel underlayer. The electroplating serves the purpose of providing solderability/brazeability to the packages along with offering corrosion protection. These machined and electroplated packages are then installed with feed-throughs by using gold-tin or gold-germanium braze alloys to braze the inside of the plated hole and outside of the Kovar or nickel-iron ring of the feed-through. An electroplated seal ring consisting of Kovar or stainless steel is also brazed to provide seam sealing capability for the cover at the same time the feed-throughs are brazed. Brazing of Kovar or the stainless steel seam sealing ring presents a problem due to voids and leaching of electroplated material at the brazing temperature, often resulting in leaky packages.
The package now is ready to accept a circuit carrying ceramic card. In some cases, the ceramic cards are soldered or attached by epoxy onto a carrier plate of intermediate expansion prior to epoxying/soldering the carrier plate inside the package. The carrier plate minimizes the effect of expansion mismatch between the package and the ceramic card and tends to prevent the warping or cracking of the ceramic card and in some cases the detachment of ceramic card from the base of the package. This is because copper and aluminum and its alloys have a high coefficient of thermal expansion (aluminum @22 ppm/.degree.C. and copper @16 ppm/.degree.C.) compared to the ceramic card at 7 ppm/.degree.C.). The disadvantages of carrier plates is in additional weight and barrier to thermal dissipation.
Patents which are relevant to the invention are as follows:
Glickman U.S. Pat. No. 3,320,351 discloses a housing to electrically isolate separate miniature circuits, one from the other.
Le Gales U.S. Pat. No. 3,826,953 discloses a case having a metallic base and cover separated from each other by a hollow insulating body to form a receptacle. An intermediary support can be located inside the receptacle and carry a number of the semiconductor devices which are added to those which are normally soldered to the base of the case.
Benjamin U.S. Pat. No. 3,936,864 discloses a microwave transistor package which will dissipate maximum power and has a ceramic mounting pad brazed to an underlying copper base and has a nickel apertured plate mounted around the mounting pad. The entire package can be hermetically sealed.
Frazee et al. U.S. Pat. No. 3,943,557 discloses a cobalt oxide humidity sensor of reduced resistivity within a hermetically sealed semiconductor package.
Scherer U.S. Pat. No. 4,266,089 discloses a flat package for micro-circuits with a copper bottom and stainless steel frame for good heat transfer.
Scherer et al. U.S. Pat. No. 4,649,229 discloses a flat package for electric micro-circuits that has an PG,8 iron-nickel-cobalt alloy frame which is brazed to a molybdemum bottom, and has successive layers of copper, nickel and gold plating.
Bigler et al. U.S. Pat. No. 4,760,440 discloses a package to mount a CCD image sensor, with a minimum amount of expansion or contraction, on a silicon substrate.
Knop et al. U.S. Pat. No. 3,614,827 is a recently expired patent which shows a particular process for explosively bonding dissimilar metals to each other.